随着可再生能源大规模接入电网，电力电子设备比例不断增加。构网型逆变器越来越受到关注，其中虚拟同步机（VSG，Virtual Synchronous Generator）控制技术使得逆变电源模拟了传统同步发电机的惯性、阻尼特性、调频和调压等外特性，能够实现对功率的无差控制，因此最具有代表性。对虚拟同步机的小干扰稳定性和暂态同步稳定性研究较多，而有关暂态稳定性分析的较少，本文的工作就是基于此展开，主要工作如下：首先，详细介绍了虚拟同步机功率环控制结构和电压电流双闭环控制结构，建立了单机并网系统全阶的 13 阶模型和等效 3 阶简化模型。先后建立了功率解耦的小信号模型、功率耦合的小信号模型、功率耦合的电磁暂态模型，逐渐提高了小信号模型的阶数，从而提高了模型的准确度，以此分析了控制参数对系统稳定性的影响，指出了“准稳态”模型不适用于分析小惯量、低阻尼系统的稳定性。其次，基于改进功角特性曲线和等面积法则给出了计算极限切除角的计算方法，对比了简化模型和详细模型下给出的极限切除时间。基于计及阻尼项的能量函数计算极限切除时间时，基于稳定域的多周期扩展给出了能量函数在多周期上的计算方法，得到了多个极限切除时间。使用李雅普诺夫能量函数分析了参数大小对系统吸引域的影响，并做出物理意义上的解释。强调了阻尼系数对虚拟同步机暂态稳定性的影响，指出存在临界阻尼系数使得系统具有全局稳定性，讨论了保守值和整定值的应用时间范围。分析了计及电流限幅环节下的暂态稳定与失稳的几种形态，提出了提升暂态稳定性的控制方法，保证限幅环节下的全局稳定性。最后提高了常用的虚拟同步机的暂态模型阶数，分析计及了无功电压环时无功积分系数对暂态稳定性的影响。最后，提出了一种基于虚拟阻抗的虚拟同步机低电压穿越控制方法，为系统提供一定的无功支撑，保证输出电流幅值不越限，同时最大化输出有功功率。在分析暂态冲击电流和改变控制回路后的稳态电流大小的基础上，给出了虚拟阻抗取值的计算方法。不对称故障下，在传统的平衡三相电流控制方式基础上改进，抑制输出频率波动，同样基于虚拟阻抗的方式进行低电压穿越，控制虚拟同步机输入电网的无功电流大小。

With the large-scale integration of renewable energy into the grid, the proportion of power electronic equipment in the power system is increasing. Grid-forming inverters are attracting more and more attention. VSG (Virtual Synchronous Generator) control technology enables the inverter power supply to simulate the external characteristics of traditional synchronous generators such as inertia, damping characteristics, frequency modulation and voltage regulation, and can realize no error control of power. Therefore, it is the most representative. There have been a lot of research on the small disturbance stability and transient synchronous stability of virtual synchronous machine, but there are few studies on transient stability analysis. The main work of this paper is introduced as follows:Firstly, the power loop control structure and the double closed-loop control mode of voltage and current of the virtual synchronous machine are introduced in detail. And a full order 13thorder model and the equivalent simplified 3rdorder model of virtual synchronous machine connected to the grid is given. A small signal model of power decoupling, a small signal model of power coupling and a electromagnetic transient small signal model of power coupling are established successively to improve the order of the small signal model thereby improving the accuracy of the model. The influence of control parameters on the stability of the system is analyzed. It is also pointed out that the quasi-steady-state model is not suitable for analyzing the stability of low inertia and low damping systems.Secondly, based on the improved power angle characteristic curve and the equal area law, a calculation method for calculating the critical clearing angle is given. The critical clearing time given by simplified and detailed models are compared. The critical clearing time is also calculated based on the energy function including damping term. Based on the multi-period extension of the stable domain, a calculation method for the energy function on multiple periods is given, and multiple critical clearing times are obtained. The influence of parameters on the attractive domain of the system is analyzed by using the Lyapunov energy function and the specific physical explanation is provided. The influence of damping coefficient on the transient stability of virtual synchronous machine is emphasized. It is pointed out that the existence of critical dampingcoefficient makes the system have global stability, and the application time range of conservative value and setting value is discussed. Several forms of transient stability and instability under current limiting link are analyzed, and a control method to improve the transient stability is proposed to ensure the global stability under current limiting. Finally, the order of transient model of virtual synchronous machine is increased byconsidering reactive voltage loop. The influence of integral coefficient in the reactive voltage loop on transient stability is analyzed. Finally, a low voltage ride through control method for virtual synchronous machine based on virtual impedance is proposed to provide a certain reactive power support for the system, ensure that the output current amplitude does not exceed the limit, and maximize the output active power. On the basis of analyzing the transient impact current and the steady-state current after changing the control loop, a calculation method for the value of the virtual impedance is given. Under asymmetrical faults, the control method is improved on the basis of the traditional balanced three-phase current control method to suppress output frequency fluctuations. Also based on the virtual impedance, low voltage ride through is performed to control the magnitude of reactive current input to the power grid by virtual synchronous machine.